A Schottky diode is a semiconductor device formed by a metal contacting a semiconductor layer. The junction between the metal and the semiconductor layer forms a rectifying junction with improved diode switching capability as compared to p-n junction diodes formed entirely in a semiconductor layer. Schottky diodes thus have lower turn-on voltages and faster switching speeds as compared to p-n junction diodes. Schottky diodes are ideal for applications where switching loss is the major source of energy consumption, such as in switch-mode power supplies (SMPS).
Electronic devices made from nitride-based compound semiconductor materials are known. Such electronic devices are also known as III-Nitride semiconductor devices as being formed from Group III nitride based materials. Nitride-based compound semiconductor devices are desirable for their wider band gap and higher breakdown voltage characteristics, rendering them suitable for high voltage and high temperature applications. In particular, III-V gallium nitride (GaN) compound semiconductor Schottky diodes having a high breakdown voltage and a low on-resistance have been described. The efficiency of switch-mode power supplies can be improved through the use of III-Nitride semiconductor Schottky barrier diodes.
III-nitride based semiconductor devices are capable of maximizing electron mobility by forming a 2-dimensional electron gas at the hetero-interface of two different III-nitrides such as AlGaN and GaN. The 2-dimensional electron gas is believed to compensate for the strain-induced piezoelectric polarization charges and spontaneous polarization charges arising from the non-ideal nature of the III-nitride crystal structure. The 2-dimensional electron gas is quantum-confined in the band bending region of a hetero junction where a narrower bandgap III-nitride (e.g., GaN) joins a larger bandgap III-nitride (e.g., AlGaN). Thus in a Schottky-like diode, electrons will flow along a confined channel between the anode electrode and the cathode electrode. The charge density is determined by hetero-structure parameters such as the Al composition, AlGaN layer thickness, and inherent crystal polarity. In III-nitride power devices, the charge density will respond to the applied gate voltage and can be removed locally according to changes in the energy band gaps. Therefore, the switching speed of III-nitride power devices can be very rapid.
FIG. 1 shows an example of a GaN-based Schottky diode. The diode 100 includes a substrate 10, a buffer layer 20, a GaN layer 30 formed on the buffer layer 20 and an AlGaN layer 40 formed on the GaN layer 30. An anode 60 and cathode 70 serve as electrical contacts for the device. The anode 60 is formed on the AlGaN layer 40 and establishes a Schottky interface therewith. The cathode 70 is formed on the GaN layer 30 and establishes an ohmic contact therewith.
One problem with Schottky diodes such as the device shown in FIG. 1 is that they often have a high forward voltage drop. This problem occurs because the conducting current has to travel through the relatively large potential barrier established by the Schottky contact on the AlGaN layer. In general, the larger the potential Schottky barrier the larger the resulting forward voltage drop. One way to ameliorate this problem is to reduce the thickness of the AlGaN layer. Unfortunately, however, this also reduces the two-dimensional electron gas underneath the AlGaN layer. For instance, the potential barrier of a Schottky contact formed by a Ni/Au metal on an AlxGaN layer is about 1.1 eV, whereas the potential barrier of a high voltage silicon-based Schottky diode is about 0.7 eV, which results in a significantly lower forward voltage drop.